Should be an easy answer to get.
isn’t it inside the config.php ?
For an ie. i have the following setup in my config file:
# define the amount of emails you want to send per period. If 0, batch processing
# is disabled and messages are sent out as fast as possible
define('MAILQUEUE_BATCH_SIZE', 0);
# define the length of one batch processing period, in seconds (3600 is an hour)
define('MAILQUEUE_BATCH_PERIOD', 1800);
Best regards,
Ruslán